Heat exchangers are normally used to extract thermal energy from a heated fluid and may be employed in a wide variety of diverse applications. One such application includes the heating of ambient air by hot waste fumes from an industrial furnace.
In general, conventional heat exchangers utilize a matrix of tubes supported on each end by what is known in the art as a tube sheet. Ambient air flows through these tubes which are disposed in a cross flow of the hot waste fumes. The ambient air in the tubes is heated by the fumes, with the heated air being used for such things as preheating process combustion air or for a wide variety of other purposes.
The most conventional type of heat exchanger employs metal tubes which are welded at their ends to a supporting metal tube sheet. Unfortunately, these metal heat exchangers are subject to deterioration especially when the hot waste fumes are at elevated temperatures and contain chemically corrosive or abrasive particles.
Heat exchangers employing ceramic components have been used in the past in these types of adverse environments. One known heat exchangers employs a sponge or matrix made of ceramic material. Unfortunately, the particulates in the waste fumes have a tendency to plug the matrix after a period of time thereby decreasing its efficiency and, in some instances, creating a fire hazard. Another known system employs metallic springs pushing against one end of the ceramic tube in an effort to provide sealing engagement between the tube and the supporting tube sheet. Unfortunately, systems employing metal components to seal ceramics are subject to leakage problems since metal has a different rate of expansion then ceramic. In addition, the metallic components are still subject to deterioration under the above mentioned adverse conditions in which these types of heat exchangers may be used.
Most of the known heat exchanger designs employ straight sided tubes which empty into plenums formed between the supporting tube sheets and the inner wall of the external housing or casing. The plenums are designed to carry the ambient air to other zones in the internal heat exchanger construction employing another set of tubes for passing the air back through the central chamber through which the heated waste fumes flow. The flow of the ambient air between the plenums and tubes creates a pressure loss within the system. These pressure losses must be overcome by an increase in the horsepower of the fans for moving the ambient air in order to maintain a given velocity of the ambient air flow. These pressure losses also make it difficult to maintain an air tight seal in the ambient air flow subsystem. The resultant leakage which may occur not only decreases the flow of the ambient air but also may allow the fumes to contaminate the ambient air. This mixture is especially undesirable when the waste fumes contain chemically corrosive, abrasive or toxic particles.